1. Field of the Invention
This invention relates to a medical multi-channel radio telemetry system for remotely monitoring patients in a hospital.
2. Description of the Prior Art
Bedside (wired) and ambulatory (wireless) telemetry systems have been used for many years to monitor the performance of the heart and other vital signs of post-coronary and other at-risk patients. While the bedside monitors have been connected to a number of electrodes to monitor various parameters, ambulatory systems have been mostly single channel due to the limitations of the FM-FM carrier system used.
Existing ambulatory systems are able to provide one signal only showing such features as temperature only, heart rate only, or the shape of the heart pulse only, but give no indication of the location of the patient. Bedside systems are traditionally connected back to the central station by means of co-ax or multi-wire cables and are thus difficult to relocate or revise.
Many existing telemetry systems operating in a hospital produce signals that are subject to multi-path cancellations at the receiving antenna and thus "dead-spots" where little or no signal is present. One method employed to overcome this problem is to use active antennas which incorporate a built-in amplifier or to use a large number of antennas placed very close to one another. Unfortunately, this method often increases the number of, or level of, signal attenuations experienced in the dead spots.
The technology of existing systems is based on analog FM-FM transmission where the analog signal being monitored is used to frequency modulate an RF carrier. Such a system, by the nature of the demodulation process, requires a signal-to-noise ratio at the receiver of at least 10db to give a useable signal. Since the output power of the transmitter is limited (due to Federal Communications Commission regulations designed to prevent interference to other users) this signal-to-noise ratio requirement limits the useful range of the signal and sets limits on the ability of such a system to work in electrically noisy hospital environments.